Method of increasing the area of a useful layer of material transferred onto a support

ABSTRACT

A composite structure that includes front faces of the first and second substrates that are molecularly bonded to each other. The dimensions of the second substrate outline are larger than the first substrate outline, and a peripheral side of the second substrate substantially borders the second front face and is oriented generally perpendicularly with respect thereto. The front faces are molecularly bonded such that the outline of the first front face is disposed at least partially within the outline of the second front face. A peripheral ring extending around the first front face and facing the first substrate, in which bonding between the front faces is weak or absent, has a maximum width of less than about 0.5 mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 10/619,596 filedJul. 16, 2003 now U.S. Pat. No. 7,294,557, which application claims thebenefit of U.S. provisional application No. 60/473,137 filed May 27,2003. The entire content of each application is incorporated herein byreference thereto.

FIELD OF THE INVENTION

The present invention relates to a method of increasing the area of auseful layer of semiconductor material which is effectively transferredonto a support during the fabrication of substrates, in particular foruse in electronics, optics, or optoelectronics.

BACKGROUND OF THE INVENTION

Currently, substrates fabricated using techniques combining bonding bymolecular bonding (known as “wafer bonding”) and transferring a usefullayer onto a support have a zone known as a “peripheral ring”. Thisperipheral ring is a zone located at the periphery of the support inwhich transfer of the useful layer has not occurred. The peripheral ringalso includes a zone in which the useful layer has been transferred onlypartially or has disappeared during subsequent treatment due to its poorbonding to the support.

Accompanying FIGS. 1 and 2 are respective cross-sectional and plan viewsof a substrate that is known to the skilled person under the acronym“SOI,” meaning “silicon on insulator”. FIG. 1 shows a support 1 ofsilicon onto which a composite layer 2, comprising a layer of siliconoxide 21 surmounted by a layer of silicon 22, has been transferred bymolecular bonding.

Peripheral ring 3 defines a substantially annular zone of support 1 ontowhich the composite layer 2 has either not been transferred or has beentransferred poorly or incompletely to a substantial level duringtransfer of the layer. In the plan view of FIG. 2, it can be seen thatthis ring 3 is typically of varying width and/or shape, i.e. that thevertical side 200 of the composite layer 2 can be irregular and jaggedIslands 2′ of composite layer may exist, which have been transferredonto the support 1, but which are spaced apart from the remainder of thecentral portion of said composite layer 2. It should be noted that FIGS.1 and 2 are diagrams that are not to scale both as regards the thicknessof the different layers and of the support and as regards the width ofthe ring.

This ring phenomenon occurs with other substrates, for example thoseknown under the acronym “SICOI” meaning “silicon carbide on insulator”or under the acronym “SOQ” meaning “silicon on quartz”. Othermulti-layer substrates, such as those comprising gallium arsenide onsilicon (AsGa/Si), also exhibit said ring.

Independently of the diameter of support 1, which typically varies, forexample, from 2 inches (50 millimeters (mm)) for silicon carbide to 12inches (300 mm) for certain silicon substrates, the ring 3 generallyvaries from about 1 mm to about 4 mm in width, typically plus-or-minus0.5 mm. Further, the width of the ring 3 can fluctuate, i.e., it can besmaller on one side of the substrate than on the other.

The appearance of the rings during layer transfer has a variety oforigins and causes, as discussed below. Certain factors that can causering 3 to appear include chamfers on the substrates used, variations inbonding energy between layer 2 and the supports, and finally certainaggressive steps in substrate fabrication methods.

In order to explain how ring 3 typically appears, reference is made tothe accompanying FIG. 3 which is a diagrammatic cross-section of aportion of the sides of two substrates bonded together by molecularbonding. A source substrate 4 is preferably a layer from which thefuture useful layer is to be cut, and a support substrate 5 is selectedto receive the useful layer. This figure illustrates the prior art. Inthe remainder of the description and drawings, the substrates areassumed to be circular in shape, as this is the shape used mostfrequently. Other shapes can alternatively be used.

The source substrate 4 has two opposite faces that are parallel in mostdesired applications. In FIG. 3 reference numeral 400 designates whatwill be referred to herein as a front face of the opposite faces. Thefront face 400 is intended and prepared for bonding onto the support 5.

The source substrate 4 has a side 41, which can be a peripheral side.Further, the substrate 4 has preferably undergone treatment that forms azone or region of weakness 42 that defines two portions: a rear portion46 and a useful layer 43. The useful layer 43 is intended for transferto the support 5. Throughout the remainder of the description, theexpression “useful layer” refers to the transferred layer. The thicknessof the useful layer typically depends on whether it is obtained, forexample, by a method of implanting atomic species or by abrasivepolishing and/or chemical etching as is described below.

In one embodiment, the substrates used both as the source substrate andas the support substrate are commercially available substratessatisfying standardized requirements (for example SEMI M1-0302 standardsfor a silicon substrate). Those standards are mainly concerned withensuring that the substrates can be accepted by the equipment of as widea range of users as possible. According to these standards, at theintersection between the side 41 and the front face 400, the substrate 4has an annular primary chamfer 44 or primary drop oriented at an angleα, which can be large and close to 45°, with the extension thereof, andmore precisely with the central zone 40, which is preferably flat to ahigh degree of precision, as will be explained below. The primarychamfer 44 extends over a width L in the radial direction parallel tothe front face 400. Width L generally varies from about 100 micrometers(μm) to about 500 μm, depending on the substrates used. The primarychamfer 44 is intended to limit the risk of mechanical breakage andnotching of the source substrate 4.

In a similar manner to that just described for the source substrate 4,the support substrate 5 also has a front face 500, a side 51 and aprimary chamfer 54 according to typically present standards. Whensubstrates 4 and 5 of the prior art are bonded to each other, bondingdoes not occur at chamfers 44 and 54 because of the magnitude of angleα. The width of the rings can thus be expected to generally correspondto the width L of the primary chamfers 44 and 54. In practice, however,the width of FIG. 3 is typically even wider.

It has been observed that the front face 400 of substrate 4 actually hastwo zones, namely a first flat zone 40 located substantially at thecenter of the substrate 4 and hereinafter also termed the “flat centralzone,” and a second zone 45 surrounding the first zone 40.

The second zone 45 is a secondary chamfer, which is generally annular,or a secondary drop, forming an angle β with the plane of the flatcentral zone 40. The secondary chamfer of second zone 45 extends betweenthe flat zone 40 and the primary chamfer 44.

Throughout the remainder of the description and claims, the expression“flat” means a flatness that is suitable for bonding. The expression“central zone” designates a zone located substantially centrally or evenat the center of the front face of the substrate and which can belocated at various degrees of eccentrically, and most preferably,located slightly eccentrically on the front face 400.

It should be noted that as FIG. 3, the following figures are onlydiagrammatic in nature and the magnitude of angle β has beenconsiderably exaggerated in the figures for clarification purposes. Moreprecisely, the secondary chamfer 45 a drop that is less sharp than theprimary chamfer 44 and that appears during the various substrate shapingsteps such as lapping, polishing, and chemical etching, which stepsproduce an etching and material-removal effect that is greater near thesubstrate side 41. The secondary chamfer 45 is presently not subject toindustry standards. Its width L′ taken in a radial direction varies fromabout 500 μm to 3000 μm on substrates that are commercially available onthe market. Further, the value of angle β also fluctuates, and thesecondary chamfer 45 is not flat in cross-section as showndiagrammatically in FIG. 3 but can be domed or irregular in places.

As a result, in practice, and in contrast to the diagrammaticrepresentation of the figures, the side of the source substrate 4 is notformed by a plurality of beveled slopes, but instead by an overallconvex shape, typically without edges between the secondary chamfer 45and the primary chamfer 44 or between the primary chamfer and the side41. The purpose of the convex shape is to avoid any nicking of thesubstrate 4.

In a manner similar to that just described for the source substrate 4,the support substrate 5 has a flat central zone 50 and a substantiallyannular secondary chamfer 55, but has similar irregularities to thesecondary chamfer 45.

Molecular bonding is a technique that does not tolerate substantialnon-planar surfaces, the existence of secondary chamfers 45, 55 resultsin poor bonding and layer transfer in the zone of these surfaces,resulting in the appearance of a peripheral ring 3. In addition, asecond reason for the appearance of the ring 3 is that the bondingenergy between two facing substrate faces fluctuates as a function ofparameters such as roughness, flatness and the chemical nature of thesurfaces in contact, the presence of particles and impurities, etc.These parameters can also vary in a less controlled manner at the sidesof the substrates, thereby also contributing to the formation of thering 3.

Finally, another possible cause for formation of the ring 3 is the useof certain aggressive or vigorous steps during the substrate fabricationmethods.

Methods of fabricating substrates known under the acronym BESOI (bondand etchback silicon on insulator) bond a source substrate onto asupport substrate, at least one of the faces of the source substratebeing coated with a layer of oxide. The exposed surface of the sourcesubstrate then undergoes an abrasive polishing and/or chemical attacketching treatment, followed by polishing until the source substratebecomes a useful layer. In this type of method involving chemical attack(with the risk of partial delamination of the bonding interface),oxidation affecting the lateral and frontal portions of the sourcesubstrate, and mechanical abrasive polishing forces, both tend toenlarge the ring 3.

Similarly, in methods involving detachment of a layer by fracture alonga zone of weakness, it has been observed that around the peripheralsides collective structure of the bonded substrates, detachment oftentends to occur at the bonding interface instead of at the zone ofweakness, resulting in the formation of an annular ring 3, sometimeswith a large surface area.

Referring again to FIG. 3, in the case in which the zone of weakness 42is formed by implantation of atomic species, such as by hydrogenimplantation, it has been observed that, during subsequent treatment todetach the useful layer 43 from the remainder of the source substrate 4,expansion of hydrogen bubbles exerts a substantially perpendicular forceon the surface of the secondary chamfer 45. In the zone of the secondarychamber 45, this force is often not compensated by sufficiently strongbonding since the secondary chamfer 55 of the support 5 is spaced fromthe secondary chamfer 45 of the useful layer 43 by an angle 213, or theaddition of the angles β if the angle of each opposing chamfer isdifferent, and the bonding is thus ruptured. Thus, bubbles are formed atthe surface of the secondary chamfer 45 instead of at the surface oredge of the layer 43 being transferred onto the support substrate 5. Inother words, bonding occurs but its quality is poor in this area.

A number of disadvantages are associated with the existence of said ring3. First, it is not possible to fabricate electronic components on thesurface of this ring 3. Unfortunately, from an economic point of view,each extra square millimeter of area can make it possible to fabricate alarger number of components per substrate. Furthermore, this ring isirregular as explained above, and its width can vary from one side tothe other of the substrate, thus giving rise to problems ofreproducibility in the various steps of an industrial process when sucha substrate is used in a production facility.

The prior art includes methods of polishing the side of a substrate soas to make it possible to eliminate the ring 3, see for example documentU.S. Pat. No. 6,221,774. A method of mechanically polishing sides isalso known as used by the supplier SEZ. This method is used on solidsilicon substrates after deposition operations, which are known to runthe risk of being associated with the effects of material being removedfrom the sides (known as “lift-off” or “peel-off”), i.e. leading to ahigh level of particulate contamination. Nevertheless, those methodstend to increase the size of the zone that has no transferred layer atthe periphery, thereby restricting the useful area. In addition,finishing operations on the ring can lead to defects at the periphery ofthe transferred layer.

SUMMARY OF THE INVENTION

The present invention relates to a method for transferring a firstsubstrate or a portion thereof to a second substrate, which arepreferably crystalline and of bulk material, to improve the bondingtherebetween. In the preferred method, first and second front faces offirst and second substrates, respectively, are bonded to each other toprovide a composite structure. A peripheral side of the second substratesubstantially borders the second front face and is oriented generallyperpendicularly with respect thereto, preferably being perpendicular orquasi-perpendicular with respect to the second front face. Preferably,the second substrate is substantially free of a primary chamfer betweenthe peripheral side and the second front face thereof, and morepreferably is substantially free of any chamfer between the peripheralside and the second front face. The second front face has dimensionslarger than the first front face, and the front faces are positionedduring bonding so at least a portion of the outline of the first frontface is disposed within the outline of the second front face.

The first front face is preferably completely disposed within theoutline of the second front face during the bonding. This is done tominimize the size of a peripheral region about the first front faceoutline and within a region of overlay between the first and secondfront faces in which the bonding between the faces is weak or absent.Preferably the width of the peripheral ring at its maximum is less thanabout 0.5 mm, more preferably less than 0.2 mm. A useful layer isprovided from a donor substrate, which comprises the first or secondsubstrate adjacent the bonded face thereof

Preferably, the useful layer is of semiconductor material. In apreferred embodiment, both the first and second substrates can be madeof a semiconductor material, most preferably at least at the frontfaces. The preferred useful layer produced is useful for producing anelectronic, optic, or optoelectronic component or substrate.

In the preferred method, the useful layer is separated from a donorportion of the donor substrate of the composite structure to provide aproduct wafer that includes the useful layer. The useful layer can beseparated from the donor portion by splitting. The splitting, in turn,can be accomplished in several manners, including by forming a zone ofweakness between the useful layer and the donor portion to facilitateand locate the splitting of the useful layer from the donor portion. Thezone of weakness may be formed, for example, by implantation of atomicspecies or by providing a porous layer. The useful layer is preferablyseparated by applying electrical or mechanical stress to, supplyingthermal energy to, or chemically etching the composite structure, or bycombinations thereof.

The first substrate can comprises a first primary chamfer extendingaround the first front face and can have a primary chamfer outline thatis at least partially disposed within the outline of the second frontface during bonding. This primary chamfer outline is preferably disposedsubstantially entirely within the second front face during bonding.Additionally, the front faces are preferably flat, and at least one ofthe front faces can comprise an insulator. With round faces orcylindrical substrates, the second front face or second substratepreferably has a diameter 0.3 mm greater than the first front face orfirst substrate.

A preferred composite structure in accordance with the inventionincludes front faces of the first and second substrates that aremolecularly bonded to each other. The dimensions of the second substrateoutline are larger than the first substrate outline, and a peripheralside of the second substrate substantially borders the second front faceand being oriented generally perpendicularly with respect thereto. Thefront faces are molecularly bonded such that the outline of the firstfront face is disposed at least partially within the outline of thesecond front face. A peripheral ring extending around the first frontface and facing the first substrate, in which bonding between the frontfaces is weak or absent, has a maximum width of less than about 0.5 mm.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Other characteristics and advantages of the invention become apparentfrom the following description of several preferred implementations ofthe invention. This description is made with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic view showing a vertical diametrical sectionthrough an SOI type substrate;

FIG. 2 is a diagrammatic plan view thereof;

FIG. 3 is a diagrammatic fragmentary, vertical section of a sourcesubstrate and a support substrate bonded together using a prior arttechnique;

FIG. 4 is a fragmentary diagrammatic view in vertical section of asource substrate and a support substrate in a first embodiment of theinvention, with the substrates shown ready for bonding together;

FIGS. 5A-5C are fragmentary diagrammatic views in vertical sectionshowing a source substrate and a support substrate in three embodimentswith the source substrate having the entire front face as a flat zoneand peripheral side faces that are perpendicular or quasi-perpendicularto the flat zone; and

FIGS. 6A-6C are fragmentary diagrammatic views in vertical sectionshowing a source substrate and a support substrate in the threeembodiments with the support substrate having the entire front face as aflat zone and peripheral side faces that are perpendicular orquasi-perpendicular to the flat zone.

FIG. 7 is a top view above of an angular portion of a support accordingto the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention reduces the area of the peripheral ring byincreasing the quantity of material taken from the source substrate thatis transferred onto a support during a layer transfer process. To thisend, the invention provides a method of increasing the area of a usefullayer of a material, in particular a semiconductor material, transferredonto a support substrate, during the fabrication of a compositesubstrate in particular for electronics, optics, or optoelectronics, themethod comprising at least one step of molecular bonding a “front” faceof a source substrate to a “front” face of a support substrate. A usefullayer from the source substrate is transferred onto the supportsubstrate with the useful layer including a portion of the sourcesubstrate extending to the front face.

In a preferred embodiment, before the bonding step, the front face ofone of the support and source substrates, which is referred to herein asthe “first” substrate, presents at least a flat central zone, the frontface of the other substrate, referred to as the “second” substrate, hasa flat zone bordered by an adjacent peripheral side face, substantiallyperpendicular or quasi-perpendicular thereto, preferably withsubstantially no, or without any, significant surface at another angledisposed between the flat zone and the peripheral side face. The outeroutline of the flat zone has dimensions larger than the dimensions ofthe outer outline of the flat central zone of the first substrate.During bonding, the substrates are applied against each other preferablysuch that the outline of the flat central zone of the first substrate isinscribed within the outline of the flat zone of the second substrate.In this manner, a larger area of a larger area of the useful layer istransferred.

According to other advantageous and non-limiting characteristics of theinvention, taken singly or in combination:

-   -   the first substrate can be surrounded by a primary chamfer. The        dimensions of the outer outline of the flat zone of the second        substrate are preferably greater than the dimensions of the        inner outline of the primary chamfer of the first substrate, and        during bonding, the substrates are applied one against the other        such a manner that the inner outline of the primary chamfer of        the first substrate is inscribed inside the outer outline of the        flat zone of the second substrate;    -   the first substrate can be bonded to the second substrate such        that its flat central zone is centered relative to the flat zone        of said second substrate;    -   the second substrate can be the source substrate;    -   prior to the step of bonding the source substrate and the        support substrate against each other, an embodiment of the        method forming a zone of weakness inside said source substrate.        The useful layer to be transferred in this embodiment extends        between the zone of weakness and the front face of this        substrate. After the bonding step, the method includes detaching        the useful layer from the remainder of the source substrate        along the zone of weakness;    -   the zone of weakness can be formed by implanting atomic species        or by providing a porous layer in respective substrate;    -   the useful layer can be detached by at least one of the        following techniques taken alone or in combination: applying        stresses of mechanical or electrical origin; supplying thermal        energy, and chemical etching; and    -   at least one of the faces of the front face of the source        substrate and the front face of the support substrate maybe        coated in a layer of an insulating material.

The invention improves the fabrication of a composite substrate. Themethod preferably includes at least one step of molecular bonding one ofthe faces of a source substrate to a facing face of a support substrate,and a step of transferring a useful layer from the source substrate ontothe support substrate. A preferred composite substrate thus includes atleast one useful layer transferred onto a support.

Referring to FIG. 4, in a first embodiment of the invention, a sourcesubstrate 6 (from which a useful layer 63 is to be taken subsequently)is ready for being molecularly bonded to a support substrate 7. In thisembodiment, the support substrate 7 can be the same as the supportsubstrate 5 described above and shown in FIG. 3. It is therefore notdescribed again in detail. The support substrate 7 presents a side 71, aprimary chamfer 74, and a front face 700 for receiving the front face600 of the source substrate 6. This front face 700 covers both a flatcentral zone 70 and a secondary chamfer 75.

As shown in FIG. 4 and in the subsequent figures, the source and supportsubstrates 6 and 7 are preferably substantially circular oncross-section. Similarly, their flat central zones are likewisesubstantially circular, and the primary and secondary chamfers (whenthey exist) are substantially annular. Nevertheless, the substrates 6and 7 and/or their respective central zones can alternatively be ofother shapes, such as oval, octagonal, rectangular, or of asymmetricalshapes within some embodiments.

As shown in FIG. 7, the secondary chamfer 75 is defined by an inneroutline C₇ and by an outer outline C′₇. It should be observed that theinner outline C₇ of the secondary chamfer 75 is also the outer outlineof the flat central zone 70 in this embodiment. Similarly, the outeroutline C′₇ of the secondary chamfer 75 the inner outline of the primarychamfer 74 in this embodiment.

In a first embodiment of the invention, shown in FIGS. 4 and 5, thefront face 600 of the source substrate 6 is not machined prior to thebonding step. Thus, compared with conventional substrates, it hasneither a primary chamfer nor a secondary chamfer. Consequently, theentire front face 600 is preferably a flat zone 67, which is surroundedby a side face 66. The outline referenced C″₆ of said flat zone 67constitutes the outer outline of the source substrate 6, and theperipheral side face 66 is perpendicular or quasi-perpendicular to saidflat zone 67 in the embodiment shown. The term “quasi-perpendicular” asused herein with reference to side face 66 means that by the side face66 is substantially perpendicular to the zone 67, even if certainsubsequent polishing treatments might sometimes modify the angle to asmall degree.

Referring to FIG. 5, in a first variant of the invention (shown incontinuous lines), when the source substrate 6 is bonded to the support7, the side face 66 of the source substrate 6 is situated vertically inregister with the side face 71 of the support 7, and the two substrates6 and 7 have substantially the same dimensions (or substantially thesame diameter in the case that the substrates are circular). Thisvariant is advantageous in that it enables traditionalsubstrate-handling equipment to be used, since the diameter of thesource substrate 6 as measured across its side face 66 is unchangedrelative to the substrates presently in use in this type of methodinvolving layer transfer and molecular bonding.

In a second variant (shown in chain-dotted lines), the side face 66 ofthe source substrate 6 is situated vertically facing or in register withthe secondary chamfer 75 (between the outlines C₇ and C′₇). In a thirdvariant (shown in phantom lines), the diameter of the source substrate 6is greater than the diameter of the support 7, and the outline C″₆ ofthe flat zone 67 then extends beyond the outside dimensions of thesupport 7.

In the above-mentioned variants, care is preferably taken to ensure thatthe dimensions of the outline C″₆ of the flat zone 67 of the sourcesubstrate 6 are greater than the dimensions of the outer outline C₇ ofthe flat central zone 70 of the support substrate 7, and that duringbonding, the source substrate 6 is applied against the support substrate7 such that the outline C₇ is inscribed within the outline C″₆. Also,the dimensions of the outline C″₆ of the flat central zone 67 of thesource substrate 6 preferably are greater than the dimensions of theinner outline C′₇ of the primary chamfer 74 of the support 7. Duringbonding, the source substrate 6 is preferably applied against thesupport substrate 7 such that the inner outline C′₇ is inscribed insidethe outline C″₆.

In this manner, the secondary chamfer 75 of the support 7 is situatedfacing the flat zone 67 of the substrate 6 and forms a small angle βrelative thereto. Bonding between these two faces is consequentlyimproved, and a larger fraction of the useful layer 63 is transferredonto the chamfer 75 compared to prior art methods. Consequently, thearea of the useful layer 63 that is actually transferred is increased,and conversely the area of the peripheral ring is decreased.

Given the usual fabrication tolerances on the substrates 6 and 7, it ispreferable in practice for the source substrate 6 to have a diameter D₆₁that is at least 0.3 mm greater than the diameter D₇₁ of the supportsubstrate 7 or than, more preferably at least 0.4 mm greater, and morepreferably 0.5 mm greater. In one embodiment, the diameter D₆₁ is lessthan 2.5 mm greater than the diameter D₇₁, more preferably less than 1.5mm greater, and most preferably less than 1 mm greater. Furthermore,when commercially-available substrates 6 and 7 are used of the kind thatare made available in the form of a range of substrates of discretelydifferent increasing diameters, it is advantageous for the substrate 6to be selected as having the diameter in the range that is the diameterimmediately larger than that of the support substrate 7. Standardizedsubstrate diameters presently available include 2 inch, 3 inch, 4 inch,5 inch, 6 inch, 8 inch, and 300 mm diameters.

Advantageously, the source substrate 6 is preferably bonded to thesupport substrate 7 so that the flat central zone 70 of the support 7 iscentered relative to the central zone 67 of the source substrate 6.Thus, the useful layer 63 as transferred is also centered orsubstantially centered relative to the support 7.

In the embodiment of FIG. 6, the source substrate 6 has a flat centralzone 60, a primary chamfer 64, and a secondary chamfer 65, while thefront face 700 of the support substrate 7 has a flat zone 77substantially directly surrounded by a peripheral side face 76 extendingperpendicularly or substantially perpendicularly.

Similarly, to the embodiment of FIG. 5, the dimensions of the outlineC″₇ of the flat zone 77 of the support 7 are greater than the dimensionsof the outer outline C₆ of the flat central zone 60 of the sourcesubstrate 6. During bonding, the source substrate 6 is preferablyapplied against the support substrate 7 such that outline C₆ isinscribed within the outline C″₇. Preferably, the dimensions of theoutline C″₇ are greater than the dimensions of the inner outline C′₆ ofthe primary chamfer 64 of the source substrate 6. During bonding thesource substrate 6 is preferably applied against the support substrate 7such that the inner outline C′₆ is inscribed inside the outline C″₇.Additionally, the substrates 6 and 7 can be centered relative to eachother.

Of the two implementations described above, the implementation of FIG.5, in which the source substrate 6 presents an edge 66 perpendicular tothe flat surface 67, is generally preferred as it facilitatesimplementing the method on an industrial scale. Whichever implementationis selected for the substrates 6 and 7, the useful layer 63 can beremoved from the source substrate 6 in a variety of manners which arenow described.

In a first variation, prior to the step of molecular bonding substrates6 and 7 to each other, a zone of weakness 62 is formed within the sourcesubstrate 6 to define and delimit the useful layer 63 subsequentlytransferred to the support 7. After bonding, the useful layer 63 isdetached from the remainder of the source substrate 6 along this zone ofweakness 62. It should be observed that FIG. 4-6 are merely diagrammaticand, for clarification purposes, the useful layer 6 is shown therein asbeing much thicker than it is in reality.

Techniques for forming the zone of weakness 62 are known to the skilledperson and are not all described in detail. Advantageously, this zone ofweakness 62 can be formed by implanting atomic species from the frontface 600. The term “implanting atomic species” means any bombardment ofatomic species, including molecular or ionic species, which canintroduce the species into a material with a maximum concentration ofsaid species located at a predetermined depth from the bombarded surface600. The molecular or ionic atomic species are introduced into thematerial with an energy that is also distributed about a maximum.

Atomic species can be implanted into the source substrate 6 using an ionbeam implanter or a plasma immersion implanter, for example. Preferably,implantation is carried out by ionic bombardment. Preferably, theimplanted ionic species is hydrogen. Other ionic species canadvantageously be used alone or in combination with hydrogen, such asrare gases.

Implantation creates the zone of weakness 62 within the bulk of thesource substrate 6 and at a mean ion penetration depth. The zone ofweakness 62 is preferably substantially parallel to the plane of thefront face 600. The useful layer 63 extends from the front face 600 tothis zone of weakness 62.

A preferred method of transferring the useful layer 63 is known asSmartCut®. The zone of weakness 62 can alternatively be provided by aporous layer obtained, for example, using the method known as ELTRAN® ofCanon, which is described in European patent EP-A-0 849 788.

After the step of molecular bonding of the faces 600 and 700, the usefullayer 63 is detached from the remainder of the source substrate 6.Detachment of the useful layer 63 is preferably performed by at leastone of the following techniques, alone or in combination: applyingconstraints of mechanical origin (inserting a blade or a jet of fluidunder pressure into the zone of weakness 62) or of electrical origin,supplying heat energy, and chemical etching. These detachment techniquesare known to the skilled person. This provides a composite substrate orproduct wafer having a useful layer 63 transferred onto a support 7.

As mentioned above, the useful layer 63 is preferably detachedhorizontally, along the zone of weakness 62. The thickness of the usefullayer 63 detached is also in part determined by a “verticalself-limitation,” which is vertically in register with the zone wherebonding to the support is of sufficient strength to transfer the usefullayer 63.

It is alternatively possible to produce the useful layer 63 by thetechnique known as “bond and etchback”, in which, after bonding thefront face 600 of the source substrate 6 onto the front face 700 of thesupport substrate 7, the rear face 601 of said source substrate 6undergoes treatment by lapping and/or etching by chemical attack,typically followed by polishing, until only the thickness correspondingto said useful layer 63 remains on the support 7. In the case of SOI(silicon on insulator) substrates, it is possible to obtain the usefullayer 63 by the BESOI mentioned above.

Examples of materials to which said method can be applied follow. Thesupport substrate 7 is preferably formed from a material that isoptionally a semiconductor and that is selected, for example, fromsilicon, a transparent substrate (such as quartz or glass, for example),silicon carbide, gallium arsenide, indium phosphide, or germanium.

Preferably, the source substrate 6 is preferably formed from asemiconductor material selected, for example, from silicon, germanium,silicon carbide, silicon and germanium alloys or “compounds” (known asSi—Ge compounds), or alloys or compounds known as III/V compounds (i.e.,compounds one element of which is from column IIIa of the periodic tableand the other is from column Va, such as gallium nitride, galliumarsenide, or indium phosphide).

Finally, it should be noted that it is possible to cover the front face700 of the support 7 with an insulating layer of the oxide type (forexample SiO₂) or of the nitride type (for example Si₃N₄). Thisinsulating layer can then be interposed between the useful layer 63 andthe support 7 after detaching said layer 63.

It is possible to cover the front face 600 of the source substrate 6with an insulating material of the type mentioned above; the transferreduseful layer 63, would then comprise two layers. It is even possible todeposit a plurality of layers onto the source substrate 6, and the term“useful layer” could then designate a stack of layers.

The substrates are preferably of bulk material that is preferablycommercially available, such as wafers sliced from ingots. An ingot istypically a mass of raw material, and the general form or shape of theingot may vary. The ingot may be generally cylindrical in shape with twosubstantially conical ends, or it may be elongate, or tubular. The ingotmay have a non-circular cross section, for example square, hexagonal, oroctagonal, with or without two pointed ends, or it may be broadlyspherical (known to the skilled person as a “boule”), or it may even bein the form of a cube. The substrates can be bulk material segmentstaken from the ingot.

When the ingot is elongate in shape, the thick segments can be cutsubstantially transversally or, in contrast, longitudinally. Typically,two pointed ends of the ingot are trimmed, and the remaining irregularlateral surface is ground and turned to obtain a cylinder of preferablycircular cross section. Next, the ingot can be cut into slices using,for example, a circular saw or a wire saw. The rondelles or slicesobtained then typically undergo finishing, which can include grinding toobtain a wafer with a uniform thickness, and then polishing at least oneof the two opposite faces to obtain a flat surface. Each wafer can beimmersed in a series of chemical baths to eliminate the dust andparticles that may still subsist on the two faces and which could be asource of subsequent pollution.

While illustrative embodiments of the invention are disclosed herein, itwill be appreciated that numerous modifications and other embodimentsmay be devised by those skilled in the art. Therefore, it will beunderstood that the appended claims are intended to cover all suchmodifications and embodiments that come within the spirit and scope ofthe present invention.

1. A composite structure, comprising: a first substrate having a firstfront face that has a first outline, the front face having a flatcentral zone that has a first central-zone outline; and a secondsubstrate having a second front face that has a second outline ofdimensions larger than first central-zone outline, the second substratehaving a peripheral side substantially bordering the second front faceand oriented generally perpendicularly with respect thereto; wherein thefirst and second front faces are molecularly bonded to each other withthe first central-zone outline disposed within the second outline in anarea of overlap, with a peripheral region of the first front faceextending around the first front face in an area in which bondingbetween the front faces is weak or absent, wherein the peripheral regionhas a maximum width of less than 0.5mm.
 2. The composite structure ofclaim 1, wherein the peripheral side is oriented perpendicularly orquasi-perpendicularly with respect to the second front face.
 3. Thecomposite structure of claim 1, wherein the first central-zone outlineis disposed within the second outline during bonding and is dimensionedfor minimizing the size of the peripheral region of the first frontface.
 4. The composite structure of claim 1, wherein the first andsecond substrates comprise a semiconductor material at least at one ofthe front faces.
 5. The composite structure of claim 1, wherein thefirst substrate comprises a first primary chamfer extending around thefirst front face and having a primary chamfer outline that is at leastpartially disposed within the second outline during bonding.
 6. Thecomposite structure of claim 5, wherein the primary chamfer outline isdisposed substantially entirely within the second outline after bonding.7. The composite structure of claim 5, wherein the front face of thesecond substrate is substantially flat.
 8. The composite structure ofclaim 1, wherein at least one of the front faces comprises an insulator.9. The composite structure of claim 1, wherein the second front face hasa diameter that is at least 0.3mm greater than the first front face. 10.The composite structure of claim 1, wherein at least one of thesubstrates is of bulk material.
 11. The composite structure of claim 1,wherein the second substrate is substantially free of a primary chamferbetween the peripheral side and the second front face thereof.
 12. Thecomposite structure of claim 11, wherein the second substrate issubstantially free of any chamfer between the peripheral side and thesecond front face thereof.
 13. The composite structure of claim 1,wherein the first substrate comprises: a zone of weakness; and a usefullayer of semiconductor material disposed between the first front faceand the zone of weakness; wherein the zone of weakness is configured forfacilitating splitting the useful layer from a donor portion of thefirst substrate.
 14. The composite structure of claim 13, wherein theregion of weakness is formed by implantation of atomic species.
 15. Thecomposite structure of claim 13, wherein the region of weakness is aporous layer.
 16. The composite structure of claim 1, wherein the firstand second front faces are substantially parallel and corresponding insurface shape, with the first front face having a first outline, thesecond front face having a second outline, and the peripheral side ofthe second substrate substantially bordering the second front face andbeing oriented generally perpendicularly with respect thereto.
 17. Thecomposite structure of claim 16, wherein the second outline hasdimensions larger than the first outline, such that at least a portionof the first outline is disposed within the second outline, and thefirst substrate comprises a primary chamfer extending around and at anangle to the first front face and having a primary chamfer outline thatis at least partially disposed within the second outline, and asecondary chamfer extending between and at an angle to each of the firstfront face and the primary chamfer.
 18. The composite structure of claim1 wherein the second outline is the outer edge of the second substrateand the second substrate has a diameter that is at least 0.3mm but lessthan 2.5mm greater than the diameter of the first substrate.
 19. Thecomposite substrate of claim 18 wherein the second substrate comprises azone of weakness; and a useful layer of semiconductor material disposedbetween the second front face and the zone of weakness; wherein the zoneof weakness is configured for facilitating splitting the useful layerfrom a donor portion of the second substrate.
 20. The compositestructure of claim 1 wherein the first and second outlines havesubstantially the same dimensions.